6 9 6 



PROTO UNGULATES 



XXVIII. 2 



many successful small herbivores, and that conversely among the 

 ungulates the hyraxes are small. The skin is often thick and a variety 

 of protective coloration schemes of spots and stripes appear, the under 

 side usually being paler to eliminate shadows. Defensive weapons such 

 as tusks or horns are common, but the problem of security often leads 

 an unaggressive animal to the development of a swift gait. For this 

 the limbs are lengthened by raising up on the toes (p. 576), producing 

 first digitigrade and then unguligrade locomotion. When this happens 



n.+ cb. 



Fig. 466. Skeletons of feet of horse and cow. 

 Lettering as Fig. 465. 



the more lateral digits, failing to reach the ground, become reduced 

 and may disappear, leaving finally the characteristic one or two. The 

 movement of the limb becomes restricted to a fore-and-aft direction, 

 and the joints assume a pulley-like form, especially characteristic in 

 the trochlea of the talus, deeply grooved in artiodactyls but markedly 

 so also in perissodactyls (Fig. 466). The carpal and tarsal bones of 

 these swift-moving animals become arranged on the so-called inter- 

 locking plan, by which each elongated metapodial thrusts up against 

 two carpals or tarsals. No movements of pronation occur, and the ulna 

 and fibula tend to be reduced and fused with the radius and tibia. 

 The hoofs themselves are a characteristic development, the terminal 

 phalanx is broadened, and the claw becomes modified to surround it, 

 while a pad forms below. The elongation of the limbs is mainly in the 

 lower sections, the humerus and femur being short. Locomotion is by 

 movement of the whole limb by the action of its proximal muscles, 

 the hind-limbs being the main propellents and the fore-limbs weight- 

 bearers, with corresponding modification of the vertebral girder (p. 

 728). The neural spines are very high above the fore-legs and the ribs 



